Device and method of spacer and trial design during joint arthroplasty

ABSTRACT

A spacer block for gathering data to be used in the balancing of the joint arthroplasty or repair and in the selection of a trial insert which includes a first body piece and a second body piece. A plurality of sensors and a processor are positioned between the first body piece and the second body piece when the pieces are assembled together to form the spacer block. A chim is removably mounted to a top surface of the second body piece, the chim is associated with the plurality of sensors and positioned in relation to the plurality of sensors such that a force exerted on the chim by a weight bearing surface is detected by the plurality of sensors.

PRIORITY CLAIM

The present application is a Continuation of U.S. patent applicationSer. No. 13/406,159, filed Feb. 27, 2012, now U.S. Pat. No. 8,656,790,which is a Continuation of U.S. patent application Ser. No. 11/393,098,filed Mar. 29, 2006, now U.S. Pat. No. 8,141,437. U.S. patentapplication Ser. No. 11/393,098, filed Mar. 29, 2006 and U.S. patentapplication Ser. No. 13/406,159, filed Feb. 27, 2012 are incorporatedherein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to joint replacement, and moreparticularly, to a spacer block used to provide data to assist inselecting the size of a trial implant.

2. Related Applications

This application incorporates by reference U.S. Patent ApplicationPublication No. 2007/0233267, entitled “Application of Neural Networksto Prosthesis Fitting and Balancing in Joints,” and U.S. PatentApplication Publication No. 2007/0239165, entitled “Device and Method ofSpacer and Trial Design During Joint Arthroplasty.”

3. Related Art

Some medical conditions may result in the degeneration of a human joint,causing a patient to consider and ultimately undergo joint replacementsurgery. The long-term success of the surgery oftentimes relies upon theskill of the surgeon and may involve a long, difficult recovery process.

The materials used in a joint replacement surgery are designed to enablethe joint to move like a normal joint. Various prosthetic components maybe used, including metals and/or plastic components. Several metals maybe used, including stainless steel, alloys of cobalt and chrome, andtitanium, while the plastic components may be constructed of a durableand wear resistant polyethylene. Plastic bone cement may be used toanchor the prosthesis into the bone, however, the prosthesis may beimplanted without cement when the prosthesis and the bone are designedto fit and lock together directly.

To undergo the operation, the patient is given an anesthetic while thesurgeon replaces the damaged parts of the joint. For example, in kneereplacement surgery, the damaged ends of the bones (i.e., the femur andthe tibia) and the cartilage are replaced with metal and plasticsurfaces that are shaped to restore knee movement and function. Inanother example, to replace a hip joint, the damaged ball (i.e., theupper end of the femur) is replaced by a metal ball attached to a metalstem fitted into the femur, and a plastic socket is implanted into thepelvis to replace the damaged socket. Although hip and knee replacementsare the most common, joint replacement can be performed on other joints,including the ankle, foot, shoulder, elbow, fingers and spine.

As with all major surgical procedures, complications may occur. Some ofthe most common complications include thrombophlebitis, infection, andstiffness and loosening of the prosthesis. While thrombophlebitis andinfection may be treated medically, stiffness and loosening of theprosthesis may require additional surgeries. One technique utilized toreduce the likelihood of stiffness and loosening relies upon the skillof the physician to align and balance the replacement joint along withligaments and soft tissue intraoperatively, i.e., during the jointreplacement operation.

During surgery, a physician may choose to insert one or more temporarycomponents. For example, a first component known as a “spacer block” isused to help determine whether additional bone removal is necessary orto determine the size of the “trial” component to be used. The trialcomponent then may be inserted and used for balancing the collateralligaments, and so forth. After the trial component is used, then apermanent component is inserted into the body. For example, during atotal knee replacement procedure, a femoral or tibial spacer blockand/or trial may be employed to assist with the selection of appropriatepermanent femoral and/or tibial prosthetic components, e.g., referred toas a tibia insert.

While temporary components such as spacers and trials serve importantpurposes in gathering information prior to implantation of a permanentcomponent, one drawback associated with temporary components is that aphysician may need to “try out” different spacer or trial sizes andconfigurations for the purpose of finding the right size and thickness,and for balancing collateral ligaments and determining an appropriatepermanent prosthetic fit, which will balance the soft tissues within thebody. In particular, during the early stages of a procedure, a physicianmay insert and remove various spacer or trial components havingdifferent configurations and gather feedback, e.g., from the patient.Several rounds of spacer and/or trial implantation and feedback may berequired before an optimal component configuration is determined.However, when relying on feedback from a sedated patient, the feedbackmay not be accurate since it is subjectively obtained under relativelypoor conditions. Thus, after surgery, relatively fast degeneration ofthe permanent component may result.

Some previous techniques have relied on using sensors that are coupledto a temporary mechanical component to collect data. In these systems,the gathered information is limited to the location of the sensors.Other systems require a physician to perform a number of different teststo obtain usable data.

SUMMARY

A spacer block is provided that includes a first body piece and a secondbody piece positioned on top of the first body piece. The first pieceincludes a plurality of sensors that measure forces, such as dynamiccontact forces, between the first and second body pieces. The spacerblock includes a processor that includes a memory. The processor isoperatively coupled to the plurality of sensors to receive datatherefrom. In one aspect, at least one chim may be positioned on top ofthe second body piece.

In another aspect, the first body piece includes a plurality ofstructurally integrated poles extending vertically upward such thatdistal ends of the poles are calibrated to be in contact with the secondbody piece. The sensors may comprise a plurality of strain gaugespositioned on the poles. The strain gauges are operatively connected tothe processor and are adapted to measure compression, tension, andbending forces between the first and second body pieces. Each pole ispositioned such that the strain gauges will measure forces between thefirst and second body pieces due to contact forces exerted on theassociated chim.

In still another aspect, the spacer block includes a transmitter that isoperatively connected to the processor. The transmitter is adapted totransmit data from the processor to ta remote receiver.

In yet another aspect, the spacer block includes a handle detachablyconnected to the spacer block for manipulation of the spacer block. Thespacer block and the handle include features to allow an electricalconnection therebetween when the handle is connected to the spacerblock. The handle may include a transmitter operatively connected to theprocessor through the electrical connection, wherein data from theprocessor is transmitted to a remote receiver, when the handle isconnected to the spacer block. Alternatively, the handle may include ahard wired connection to a receiver such that data from the processorcan be sent to the receiver, through the handle, when the handle isconnected to the spacer block.

In still another aspect, the spacer block includes a handle that isintegrally formed with the spacer block. Similarly to the detachablehandle, the integrally formed handle may include a transmitteroperatively connected to the processor, wherein data from the processoris transmitted to a remote receiver. Alternatively, the handle mayinclude a hard wired connection to a receiver such that data from theprocessor can be transmitted to the receiver, through the handle.

Other systems, methods, features and advantages of the invention willbe, or will become, apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below may be better understood with reference to thefollowing drawings and description. The components in the figures arenot necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention. Moreover, in the figures,like referenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a diagram of a system that monitors forces between two bearingsurfaces.

FIG. 2 is a block diagram of monitoring circuitry.

FIG. 3 is a diagram of a system that monitors forces between bearingsurfaces.

FIG. 4 is bottom perspective view of the system that monitors forcesbetween bearing surfaces of FIG. 3.

FIG. 5 is a diagram of a closure mechanism used in a system thatmonitors a force between bearing surfaces.

FIG. 6 is a sensor configuration diagram.

FIG. 7 is a second sensor configuration diagram.

FIG. 8 is a third sensor configuration diagram.

FIG. 9 is a fourth sensor configuration diagram.

FIG. 10 is a diagram of a system that monitors a force between bearingsurfaces incorporated into a prosthetic knee joint.

FIG. 11 is a flowchart of a system that monitors a force between bearingsurfaces.

FIG. 12 a is a schematic of structures used in a force monitoringsystem.

FIG. 12 b is a second schematic of structures used in a force monitoringsystem.

FIG. 12 c is a third schematic of structures used in a force monitoringsystem.

FIG. 12 d is a fourth schematic of structures used in a force monitoringsystem.

FIG. 12 e is a fifth schematic of structures used in a force monitoringsystem.

FIG. 12 f is a sixth schematic of structures used in a force monitoringsystem.

FIG. 12 g is a seventh schematic of structures used in a forcemonitoring system.

FIG. 12 h is an eighth schematic of structures used in a forcemonitoring system.

FIG. 12 i is a ninth schematic of structures used in a force monitoringsystem.

FIG. 12 j is a tenth schematic of structures used in a force monitoringsystem.

FIG. 12 k is an eleventh schematic of structures used in a forcemonitoring system.

FIG. 12 l is a twelfth schematic of structures used in a forcemonitoring system.

FIG. 12 m is a thirteenth schematic of structures used in a forcemonitoring system.

FIG. 12 n is a fourteenth schematic of structures used in a forcemonitoring system.

FIG. 12 o is a fifteenth schematic of structures used in a forcemonitoring system.

FIG. 13 a is a table providing example dimensions for the structures ofFIG. 12.

FIG. 13 b is a second table providing example dimensions for thestructures of FIG. 12.

FIG. 13 c is a third table providing example dimensions for thestructures of FIG. 12.

FIG. 13 d is a fourth table providing example dimensions for thestructures of FIG. 12.

FIG. 14 is an alternative configuration for a force monitoring system.

FIG. 15 is a second alternative configuration for a force monitoringsystem.

FIG. 16 is a third alternative configuration for a force monitoringsystem.

FIG. 17 is an exploded view of a spacer block of the present invention,incorporating load cells as sensors.

FIG. 18 is an exploded view of a spacer block of the present disclosureincorporating strain gauges as sensors.

FIG. 18A is an enlarged portion of FIG. 18.

FIG. 19 is an exploded view similar to FIG. 18 from an angle showing anunderside of a second body piece.

FIG. 20 is a perspective view of a spacer block having an integrallyformed handle.

FIG. 21 is an exploded view of a spacer block having a detachablehandle.

FIG. 22 is an exploded view of a portion of a spacer block havingdetachable handle of another embodiment.

FIG. 23 is a plan view of a human knee having a spacer block of thepresent disclosure placed between a femur and a tibia.

FIG. 24 is a block diagram depicting various components of a jointprosthesis fitting and balancing system.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a system 100 that monitors one or more forcesbetween bearing surfaces, such as one or more dynamic contact forces,such as the force at contact between two bearing surfaces, or a dynamiccontact force related measurement such as a strain, stress, torsion,and/or pressure. An enclosure 102 may comprise first body piece 104(e.g., lower block) and second body piece 106 (e.g., upper block), eachbody piece comprising an inner surface and an outer surface. The firstbody piece and second body piece 104 and 106 are configured to matetogether. The inner surface of first body piece 104 may comprise one ormore protrusions, such as poles, posts, or beams, which are preferablyintegrally formed and which extend from a bearing surface. A portion ofthe inner surface of first body piece 104 may be recessed to receivesome or all of monitoring circuitry 108. Alternatively, first body piece104 may not include a recessed portion and monitoring circuitry 108 mayoverlay the inner surface of first body piece 104. One or more portionsof the inner surface of second body piece 106 may be recessed to receivethe one or more poles of first body piece 104 and monitoring circuitry108 when the mating body pieces are fit together.

When fit together, the external surfaces of first body piece and secondbody piece 104 and 106 may comprise the bearing surfaces of forcemonitoring system 100. As forces are applied to or removed from thebearing surfaces, a force representing the algebraic summation of one ormore forces, may be transferred from the bearing surfaces to one or moreof the poles. The transferred force may cause a measurable mechanicalmotion in one or more of the poles, such as a displacement and/ordeformation. The mechanical motion may comprise rotational motion and/orcompression and/or expansion in the longitudinal and/or latitudinaldirection of the one or more poles. Monitoring circuitry 108 sensesand/or models and/or analyzes the mechanical motion of the one or morepoles. Data representative of the mechanical motion of the one or morepoles may be used to determine whether modifications to the structure(s)exerting the sensed and/or modeled and/or analyzed dynamic contact forceis required.

FIG. 2 is a block diagram of monitoring circuitry 108. Monitoringcircuitry 108 comprises one or more sensors 200, programmableconditioning logic 202, and computer 204. The one or more sensors 200may be single or multifunction sensors that are disposed on the surfaceof one or more of the poles and which detect or measure a mechanicalmotion resulting from the application of dynamic contact forces onenclosure 102. The one or more sensors 200 convert the detected ormeasured motion into an electrical signal in a time period which occursat or near the same rate of time perceived by a human, such asreal-time. Alternatively, the one or more sensors 200 may covert thedetected or measured motion into an electrical signal in batches suchthat the conversions occur in delayed time. The electrical signal mayhave an amplitude that varies with the amount of displacement and/ordeformation of the one or more posts. Alternatively, the electricalsignal may comprise a discontinuous stream of on/off pulses.

The sensed electrical signals generated by the one or more sensors 200may be supplied to conditioning logic 202 through a signal medium, suchas a flexible signal medium. The flexible signal medium may comprise aplurality of conductors affixed to or enclosed within a continuousbendable material, such as a flexible printed circuit. The flexiblesignal medium may overlay some or the entire inner surface of first bodypiece 104, and may move relative to the movement of first body piece 104and/or second body piece 106. Alternatively, the sensed electricalsignals may be supplied to control logic 202 through a discrete wiredsignal medium and/or a wireless signal medium.

The sensed electrical signals generated by the one or more sensors 200may be conditioned by conditioning logic 202 to improve the manner inwhich the signal content is further processed by monitoring circuitry108 and/or to improve the quality of the corresponding data content.Signal conditioning may include selecting two or more signals receivedfrom sensors 200 and combining the selected signals into a singlechannel (e.g., multiplexing the received signals one after another intoa serial signal) and/or perform logic operations on all or a portion ofthe received signal (e.g., converting a voltage and/or current signalinto data representing an amount of displacement/deformation, or passingthe received signal through a Wheatstone bridge) and/or removing acontinuous noise signal from the received electrical signal (e.g.,filtering) and/or enlarging the waveform of the received intermediatesignal or signals (e.g., amplification). The signal conditioning logic202 may comprise hardware and/or software that is capable of running onone or more processors in conjunction with one or more operatingsystems.

Computer 204 may be configured to receive data, such as sensedelectrical signals, directly from sensors 200 or to receive aconditioned signal. Computer 204 may comprise processor 206, memory(volatile or non-volatile) 208, and/or transceiver 210. Processor 206may vary in size and performance depending on the tasks. Processor 206may perform control operations by transmitting control signals toconditioning logic 202. Control operations may comprise determiningwhich electrical signals are multiplexed, and/or altering an amount ofnoise attenuation, and/or varying an amplifier gain factor. The signalsreceived at processor 206 may be stored in memory 208 without undergoingany additional processing by processor 206 (e.g., raw data).

Alternatively, processor 206 may perform in real-time or delayed timearithmetic and/or logic operations on the received signals to model theforces between the bearing surfaces. The modeled data may be used todetermine the magnitude of a dynamic force exerted on differentlocations of enclosure 102 under different conditions, or to determinewhere on the enclosure a dynamic contact force is exerted. The modeleddata may be stored in memory 208 via a bidirectional bus.

Transceiver 210 is configured to receive from processor 206 data, suchas modeled data and/or sensed electrical signals, and forward thereceived data to a data gathering device 212. Data gathering device 212may be a fixed device, such as a computer, or a mobile device, such as ahandheld computer, personal digital assistant (“PDA”), and/or a mobilecommunications device. Prior to transmitting data, transceiver 210 maytransmit a control message to data gathering device 212. The message mayinform data gathering device 212 that data will be transmitted. Datagathering device 212 may then acknowledge its receipt of the controlmessage by sending an acknowledgement message back. The acknowledgementmessage may inform transceiver 210 to begin transmitting data.

Transceiver 210 may comprise a port configured to receive a transmissionwire and transmit/receiver data sequentially or simultaneously throughmultiple protocols. These protocols may include Extensible MarkupLanguage (“XML”), Hyper Text Transfer Protocol (“HTTP”), TransmissionControl Protocol/Internet Protocol (“TCP/IP”), as well as other publicor proprietary protocols developed in house or by others. Transceiver210 may additionally be coupled to an antenna and communicate with datagathering devices through wireless protocols. These protocols mayinclude 802.11b, 802.11j, 802.11g, other 802 wireless protocols,Bluetooth®, Zigbee®, or other developing wireless protocols. Based onthe modeled data, processor 206 may control the frequency with whichtransceiver 210 forwards data to a data gathering device.

FIG. 3 illustrates an exemplary system 300 that may be used as aprosthetic joint spacer block, trial, or insert used to monitor adynamic contact force between two bearing surfaces, such as between thefemur and tibia bones. Force monitoring system 300 comprises enclosure302 having a first body piece 304 and a second body piece 306. The firstbody piece and second body piece 304 and 306 may generally have aU-shape, although other shapes may be used. The outer surfaces of bodypieces 304 and/or 306 may be substantially flat or may be customized toreceive a particular contact surface. In FIG. 3, second body piece 306is customized to receive a particular contact surface. Recessed portion320 is configured with concave surfaces to receive the rounded bumps ofa reconstructed femur bone, the artificial medial and lateral condyles.The inner surface of second body piece 306 may comprise one or morerecesses configured to receive poles 308 of first body piece 304, suchthat first body piece and second body piece 304 and 306 may be matedtogether.

In FIG. 3, a plurality of poles 308 are symmetrically spaced on theinner surface of first body piece 304. Poles 308 are aligned with theunderside of second body piece 306. Where second body piece 306 includesa recessed exterior surface, such as to receive condyle portions, poles308 are aligned with the underside of the plane which passes through thelowest point of the second body piece's 306 condyle recesses. Poles 308are configured such that they contact, without detectible load transfer,the underside of the condyle recesses when no force is applied to themated first and second body pieces 304 and 306. Application of a Forceto first and/or second body pieces 304 and 306 may cause the second bodypiece 306 to push against the poles 308 thereby causing the poles 308 toundergo a detectible mechanical motion. Although poles 308 areillustrated as having a flat upper surface, poles 308 may be configuredwith a curved surface that mirrors a curved inside surface of secondbody piece 306 such that there is a uniform distance between the insidesurface of second body piece 306 and the top of poles 308.

Poles' 308 geometry relates to an applied strain, and relatedmeasurements, as the strain is dependent upon the cross-sectional areaof poles 308. Poles' 308 geometry includes a plurality of grooves intowhich one or more sensors 310 may be disposed. The one or more sensors310 may be disposed on the surface of poles 308 within the groovedportions. The distance from the base of corresponding pole may forexample be about 3 mm. Placement of the one or more sensors 310 withinthe pole grooves may protect the one or more sensors 310 when first bodypiece and second body piece 304 and 306 are mated together.Additionally, when the body pieces are mated together, a space may existbetween the non-grooved portion of pole 308 and a correspondingreceiving recess to permit the first and/or second body pieces 304 and306 to freely move. This space, for example, may be about 0.015 inches.

One or more sensors 310 may comprise a plurality of strain gages adaptedto generate a voltage in response to dynamic contact forces transferredfrom the bearing surfaces to poles 308. Strain gauges are configured tomeasure an amount of deformation of a body due to an applied force. Morespecifically, strain gauges are configured to measure or detect a changeof length in a body with respect to the original length of that body.Depending on the number of sensors 310 disposed on a pole 308 and theorientation of the sensors 310, a compression, extension, rotation,and/or bending of a pole 308 may be detected. Data detected or measuredby sensors 310, representing a compression; extension; rotation; and/orbending of a pole 308, may be provided to conditioning logic 312 throughsignal medium 314. Signal medium 314 may generally conform to the shapeof first body piece 304 but with a smaller length and width. Signalmedium 314 may comprise a plurality of conductors affixed to a flexiblecontinuous bendable material such as a flexible printed circuit. Aplurality of holes or openings may be provided in signal medium 314through which poles 308 may be received. Signal medium 314 may lieloosely on top of the inner surface of first body piece 304, itsposition being maintained by poles 308 and the remainder of themonitoring circuitry. Alternatively, the signal medium may be affixed tothe inner surface of first body piece 304 in a few locations such as topermit signal medium to flex relative to the movement of first assembly304 and/or second assembly 306.

Conditioning logic 312 may comprise hardware connected through a printedcircuit board. Conditioning logic 312 may communicate with flexiblesignal medium 314. To allow for easy assembly and/or removal, aconnector may couple conditioning logic 312 to flexible signal medium314. For example, an OMRO 0.5-pitch Lock FPC Connector may be affixed tothe printed circuit board and used to couple conditioning logic 312 toflexible signal medium 314.

In FIG. 3, one sensor is affixed to each groove of poles 308 (e.g. atotal of 16 sensors), although more or less sensors or poles could beused. Conditioning logic 312 processes the data through a multiplexer,such as a 16-channel multiplexer, and routes the multiplexed data to aseries of amplifiers. To increase the amplification range, amplifiersmay be cascaded in series. For example, a set of two amplifiers eachwith a gain factor of 100 may be cascaded in series to achieve a totalgain factor amplification of 10000. After amplification, the data iscollected by computer 316 where it may be stored or transmitted to adata gathering device. An antenna may be disposed around the peripheryof the printed circuit board and is preferably provided to transmit andreceive data.

Computer 316 may comprise battery 318. Battery 318 provides power tocomputer 316, conditioning logic 312, and/or sensors 310. In thisembodiment, computer 316 transmits and receives data to/fromconditioning logic 312 via a bidirectional bus. The transmission andreceipt of data between computer 316 and conditioning logic 312 mayoccur sequentially or simultaneously.

FIG. 4 is a bottom perspective view of force monitoring system 300. InFIG. 4, recesses 400 are integrally formed in second body piece 306 forreceiving poles 308 of first body piece 304. A central recess 402 isintegrally formed in second body piece 306 for receiving monitoringcircuitry 108.

To keep first body piece and second body piece 304 and 306 matedtogether each body piece may be configured to receive one or morefasteners along its outer rim. In FIG. 5, a plurality of screw fasteners502 are used to secure first body piece and second body piece 304 and306. The plurality of screw fasteners 502 may be configured to allowsecond body piece 306 to move downward while limiting its upward motionto about its position when no force is exerted against the body pieces.Alternatively, instead of screw fasteners 502, first body piece andsecond body piece may be configured with mating snap latches on theirout rim. The snap latches may operate in a similar manner to the screwfasteners 502—permitting second body piece 306 to move downward whilelimiting its upward motion.

The orientation of the sensors along with the principles of beam theorymay be utilized to collect data representing the mechanical motion ofthe one or more poles 308 when the first body piece and second bodypiece are force against one another. FIG. 6 is a sensor configurationdiagram for measuring or detecting bending. This configuration may beused to measure or detect the bending of a pole 308 of force monitoringsystem 300. To detect bending, two or more sensors are mounted onopposite sides of a pole in a plane that is perpendicular to an appliedforce, F_(v). In FIG. 6, two sensors are disposed on each side of pole602, 1 and 3 and 2 and 4, respectively. The inclusion of extra sensorsincreases the accuracy of the measured or detected bending. As pole 602bends, sensors 1 and 3 and 2 and 4 are stressed and generate a signalthat may be analyzed to determine the amount of movement of pole 602.

FIG. 7 is a sensor configuration diagram for measuring or detecting adeformation along a pole's axis, such as an expansion or contraction.This configuration may be used to measure or detect an expansion orcontraction of a pole 308 of force monitoring system 300. Axialdeformation may be measured or detected by disposing one or more sensorson a pole 702 in a plane parallel to a force, F_(A). In FIG. 7, sensors1 and 3 are oriented to measure or detect an axial deformation in theX-direction. Alternatively or additionally, one or more sensors may bedisposed on pole 702 in a plane perpendicular to force F_(A) and used tomeasure or detect an expansion or contraction in the Y-direction. InFIG. 7, sensors 2 and 4 are oriented to measure or detect this type ofexpansion or contraction.

FIG. 8 is a sensor configuration diagram for measuring or detecting astrain that produces a distortion or deformation of pole 802 without avolumetric change, such as a shear strain. This configuration may beused to measure or detect a shear strain of a pole 308 of forcemonitoring system 300. To measure shear strain, two sensors are disposedon pole 802 at an angle with respect to one another, such as about a 45°angle. When a measurement of the sensors is compared to a priormeasurement and it is determined that the pole has been deformed in morethan one direction, a shear force has been detected. In addition tomeasuring shear strain, the configuration illustrated in FIG. 8 may beused to measure or detect an axial or bending strain component.

FIG. 9 is a sensor configuration diagram for measuring or detecting astrain that produces a rotation or twisting action, such as a torsionalstrain. This configuration may be used to measure or detect a torsionalstrain of a pole 308 of force monitoring system 300. Torsional strainmay be measured or detected using a similar sensor configuration as thatused to measure or detect a shear strain. Accordingly, two sensors aredisposed on post 902 at an angle with respect to one another, such asabout a 45° angle. As shown in FIG. 9, torsional strain measures arotational force about a central axis. Although FIGS. 6-9 describeconfigurations for measuring or detecting individual strains, multiplesensors disposed in multiple configurations may be disposed on one ormore poles to measure or detect one or a combination of bending, shear,and/or torsional strains.

FIG. 10 is a diagram of a system that monitors a force between bearingsurfaces in a prosthetic joint environment. In FIG. 10, human knee 1000comprises femur 1002, patella 1004, tibia 1006, a plurality of ligaments(not shown), and a plurality of muscles (not shown). In this example,the prosthesis used during a total knee arthroplasty (TKA) procedurecomprises femoral component 1008 and tibial component 1010. Tibialcomponent 1010 may comprise tibial tray 1012 and force monitoring systemenclosure 1014. Force monitoring system enclosure 1014 may betemporarily attached to tibial tray 1012. Alternatively, the enclosuremay be integrally formed to provide bearing surfaces. Force monitoringsystem enclosure 1014 may comprise embedded circuitry and one or moresensors that are capable of acquiring data. The acquired data relates todynamic contact forces and the location of the dynamic contact forcesimposed upon force monitoring system enclosure 1014.

The materials used in a knee joint replacement surgery are designed toenable the joint to mimic the behavior of a normal knee. Femoralcomponent 1008 may comprise a metal piece that is shaped similar to theend of a femur, e.g., having condyles 1016. Condyles 1016 are disposedin close proximity to a bearing surface of force monitoring systemenclosure 1014, and preferably fit closely into corresponding concavesurfaces of enclosure 1014. In preferred embodiments, femoral and tibialcomponents 1008 and 1010 comprise several metals, including stainlesssteel, alloys of cobalt and chrome, titanium, or another suitablematerial. Plastic bone cement may be used to anchor permanent prostheticcomponents into femur 1002 and tibia 1006. Alternatively, the prostheticcomponents may be implanted without cement when the prosthesis and bonesare designed to fit and lock together directly, e.g., by employing afine mesh of holes on the surface that allow the femur 1008 and tibia1006 to grow into the mesh to secure the prosthetic components to thebone.

As shown, femoral component 1008 preferably resides in close proximityto an exterior surface of force monitoring system enclosure 1014.Contact between femoral component 1008 and the exterior surface ofenclosure 1014 generates a force exerted on enclosure 1014. The exertedforce is transferred to one or more poles 308 (internal to enclosure1014) and results in a deformation of one or more of poles 308. One ormore sensors 310 embedded within enclosure 1014 sense the deformationand generate a representative output signal.

FIG. 11 is a flow diagram of a system that monitors a force betweenbearing surfaces. At act 1100, a monitoring device comprising at leastone sensor and monitoring circuitry which is preferably embedded in themonitoring device is placed within a weight bearing structure. The atleast one sensor may be disposed on an internal portion of themonitoring device's structure, such as a post. At act 1102, at least onesensor detects a mechanical motion in a portion of the monitoringdevice's structure, such as a mechanical motion of an internal post. Themechanical motion is preferably detected in real-time, and may indicatea rotational motion and/or a compression or extension in thelongitudinal and/or latitudinal direction of the monitoring device'sstructure. At least one sensor generates an electrical signal, such as avoltage, responsive to the detection of the mechanical motion.

To assure a good quality measurement, the generated electrical signal ispreferably conditioned at act 1104. In an example, conditioning of theelectrical signal comprises combining one or more signals from at leastone of the sensors, substantially attenuating a noise signal, convertingthe received electrical signal into a data representative of an amountof displacement/deformation of the monitoring device's structure, and/ormultiplying the representative data signal by a static or variable gain.

At act 1106 the conditioned signal is processed by a computer.Processing the data may include performing arithmetic and/or logicoperations on the conditioned data to model the force imposed on thebearing surfaces of the monitoring device. The modeled data ispreferably stored in memory. The memory may be internal or external tothe processing computer. The processing computer accesses the datastored in the memory to perform a statistical analysis.

The data modeled by the computer, the data representing the statisticalanalysis, and/or the conditioned data prior to any processing at act1106 (e.g., raw data) may be transmitted at act 1108. The data may betransmitted to a data gathering device through a wired or wirelessmedium.

Some or all of the method of FIG. 11, in addition to the other methodsdescribed above, and/or neural network analyses as described inapplicant's published U.S. Patent Application (No. 2007/0233267),entitled “Application of Neural Networks to Prosthesis Fitting andBalancing in Joints,” may be encoded in a signal bearing medium, acomputer readable medium such as a memory, programmed within a devicesuch as one or more integrated circuits, or processed by a controller ora computer. If the method is performed by software, the software mayreside in a memory resident to or interfaced to computer 316. Preferablythe memory includes an ordered listing of executable instructions forimplementing logical functions. A logical function may be implementedthrough digital circuitry, through source code, through analogcircuitry, or through an analog source such as through an electrical,audio, or video signal. The software may be embodied in anycomputer-readable or signal bearing medium, for use by, or in connectionwith an instruction executable system, apparatus, or device. Such asystem may include a computer-based system, a processor-containingsystem, or another system that may selectively fetch instructions froman instruction executable system, apparatus, or device that may alsoexecute instructions.

A “computer-readable medium,” “machine-readable medium,”“propagated-signal medium,” and/or “signal-bearing medium” comprise anymeans that contains, stores, communicated, propagated, or transportssoftware for use by or in connection with an instruction executablesystem, apparatus, or device. The machine-readable medium mayselectively be, but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. A non-exhaustive list of examples of amachine-readable medium would include: an electrical connections (e.g.,electronic) having one or more wires, a portable magnetic or opticaldisk, a volatile memory such as a Random Access Memory “RAM”(electronic), a Read-Only Memory “ROM” (electronic), an ErasableProgrammable Read-Only Memory (EPROM or Flash Memory) (electronic), oran optical fiber (optical). A machine-readable medium may also include atangible medium upon which software is printed, as the software may beelectronically stored as an image or in another format (e.g., through anoptical scan), then compiled, and/or interpreted or otherwise processed.The processed medium may then be stored in a computer and/or machinememory.

In some force monitoring systems, computer 316 may comprise a MICA2DOTsystem that further comprises an ATMEGA128 microcontroller, a ChipconCC1000 radio, a Panasonic ERT-J1VR103J thermistor, and Flash MemoryAT45DB041 memory. The MICA2DOT system may be programmed to operate witha TINY operating system. Other force monitoring systems may usealternative shapes for poles 308, such as cylinders, hexagons,rectangles, or other polygons. The force monitoring system of thepresent disclosure may be manufactured according to different sizespecifications. In FIGS. 12 a-12 o, dimensions, in inches, are providedfor an exemplary configuration of first body piece and second body piecefor a spacer block/trial/insert (size 4 thickness 10 mm) which may beused in a TKA. Other spacer block/trial/insert sizes may range from size1-6. For example, a spacer block/trial/insert may be categorized as asize 1.5 thickness 10 mm (e.g. SIZE_(—)1P_(—)5_TH10), size 3 thickness10 mm (e.g. SIZE_(—)3_TH10), or size 6 thickness 10 mm (e.g.SIZE_(—)6_TH10). Additionally, size 1.5, 3, and 6, inserts may beconfigured with different thicknesses (measurements in mm), such asabout 8, about 10, about 12.5, about 15, about 17.5, or about 20. FIGS.13 a-13 d are tables providing examples of dimensions, for thestructures shown in FIGS. 12 a-12 o, that change according to the sizeor thickness of the body pieces.

FIG. 14 is an alternative configuration for a force monitoring system.In FIG. 14, a plurality of poles 1402 are affixed to a unitary base 1408instead of being integrally formed in first body piece 1404. Unitarybase 1408 is configured to be received in a recessed portion of firstbody piece 1404. Moreover, in the event that one or more of poles 1402or one or more sensors affixed to a pole 1402 (not shown) requiresmaintenance, or a user wishes to add additional sensors and/or poles1402, first body piece and second body piece 1404 and 1406 may beun-mated from one another by first disengaging snap latches 1418 and thedesired maintenance or changes may then be performed.

FIG. 15 is a second alternative configuration for a force monitoringsystem. In FIG. 15, a plurality of poles 1502 are affixed to a base 1508instead of being integrally formed in first body piece 1504. Base 1508is configured to be received in a recessed portion of first body piece1504. Moreover, in the event that one or more of poles 1502 or one ormore sensors affixed to a pole 1502 (not shown) requires maintenance, ora user wishes to add additional sensors and/or poles 1502, first bodypiece and second body piece 1504 and 1506 may be un-mated from oneanother by first disengaging snap latches 1510 and the desiredmaintenance or changes may then be performed. FIG. 15 differs from FIG.14 in that changes may be performed without having to disconnect sensorconnections or connections to additional hardware components (not shown)for elements whose configuration or operation is not being changed.

FIG. 16 is a third alternative configuration for a force monitoringsystem. In FIG. 16, a plurality of removable poles 1602 are affixed tofirst body piece 1604 instead of being integrally formed in first bodypiece 1604. In the event that one or more of poles 1602 or one or moresensors affixed to a pole 1602 (not shown) requires maintenance, or auser wishes to add additional sensors and/or poles 1602, first bodypiece and second body piece 1604 and 1606 may be un-mated from oneanother by first disengaging snap latches 1610 and the desiredmaintenance or changes may then be performed. FIG. 16 differs from FIG.15 in that changes may be performed to an individual pole 1602, oradditional single poles may be added without having to disconnect sensorconnections or connections to additional hardware components (not shown)for elements whose configuration or operation is not being changed.

As an alternative to the force monitoring system disclosed above, someforce monitoring systems may include a fully integrated structure. Inthese systems, the enclosure containing measurement posts and monitoringcircuitry may be formed in layers, such as by injection molding. As theenclosure is being formed (molded), the monitoring circuitry may beinserted and sealed within the enclosure.

During a surgical procedure, and prior to the insertion of a forcemonitoring system enclosure (e.g., 1014) that will remain in a jointafter the procedure is concluded, a spacer block may be inserted intothe joint to gather data and assist the surgeon in determining whetheradditional bone must be removed and in selecting the appropriate trialinsert or force monitoring system enclosure (e.g., 1014) that should beinserted into the joint. In some systems, the spacer block inserted intothe joint may be a force monitoring system enclosure as described above.A first alternative spacer block is described with reference to FIG. 17.FIG. 17 illustrates an exemplary spacer block 1730 that may be used inconjunction with a knee joint. While illustrated with respect to a kneejoint, spacer blocks may be used in conjunction with other joints.

In FIG. 17, the spacer block 1730 includes a first body piece 1732, asecond body piece 1734 positioned on top of the first body piece 1732,and at least one chim 1736 positioned on top of the second body piece1734.

As shown, in FIG. 17, two chims 1736 are mounted on top of the secondbody piece 1734. The chims 1736 are removably mounted onto the secondbody piece 1734 to allow easy replacement of the chims 1736. The chims1736 come in various thickness, and through trial and error, chims 1736having the proper thickness can be inserted to ensure that the datacollected by the spacer block 1730 is accurate. As shown, the secondbody piece 1734 includes recesses 1738 formed in a top surface 1740thereof. The chims 1736 have corresponding projections (not shown)extending from a bottom surface 1742 thereof, that engage the recesses1738 of the second body piece 1734 to secure the chims 1736 thereon.

The first body piece 1732 includes at least one sensor to measure forcesbetween the upper and first body pieces 1732, 1734. A processor 1744having a memory is mounted within the second body piece 1734 and isoperatively connected to the sensors when the upper and first bodypieces 1732, 1734 are assembled.

As shown in FIG. 17, a plurality of load cells 1746 are positionedwithin the first body piece to measure compression, tension, and bendingforces between the upper and first body pieces 1734, 1732. The loadcells are operatively connected to the processor 1744 so informationrelated to the forces between the upper and first body pieces 1734, 1732can be sent to the processor. At least one load cell 1746 is associatedwith each chim 1736.

Shown in FIG. 17, the first body piece 1732 includes two loads cells1746 for each chim 1736. The load cells 1746 are positioned below thechims 1736 such that the load cells 1746 will measure forces between theupper and first body pieces 1734, 1732 due to forces exerted on the chim1736 positioned above. More loads cells 1746 will allow more data to begathered regarding the forces on the chims 1736. Ultimately, the numberof load cells 1746 used depends on the particular application.

FIG. 18 is yet another configuration of a spacer block. Spacer block18110 includes chims 18136, a second body piece 18134, and a first bodypiece 18132 similar to those described above. As shown in FIG. 18, thefirst body piece 18132 includes a plurality of poles 1848 extendingvertically upward in relation to first body piece 18132.

Referring to FIG. 19, the second body piece 18134 includes a pluralityof pockets 1949 formed therein. The pockets 1949 are sized toaccommodate the poles 1848 from the first body piece 18132. Whenassembled, the poles 1848 will be positioned in contact with the secondbody piece 18134 within the pockets 1949. There is no pre-load betweenthe second body piece 18134 and the poles 1848, but any deflection ofthe second body piece 18134 will cause the second body piece 18134 topush against, and cause deflection of the poles 1848.

The poles 1848 have flat surfaces 1850 formed on the sides.Alternatively, grooves or slots could also be formed within the sides ofthe poles 1848. As shown in FIG. 18A, a plurality of strain gauges 1852are positioned on the flat surfaces 1850 of the poles 1848 to measurecompression, tension, and bending forces experienced by the poles 1848due to contact from the second body piece 18134.

The size of the pockets 1949 formed in the second body piece 18134 isprecisely calibrated to allow deflection of the poles 1848 and to ensurethat when the second body piece 18134 and the first body piece 18132 areassembled, and the poles 1848 are inserted within the pockets 1949, thestrain gauges 1852 are not damaged. The flat sides 1850, grooves, orslots formed on the poles 1848 provide a flat surface onto which thestrain gauges 1852 can be mounted, and provide a recessed area toprotect the strain gauges from damage.

The second body piece 18134 further includes a larger pocket 1954 formedto accommodate a processor 18144. The strain gauges 1852 are operativelyconnected to the processor 18144 via a printed circuit board or signalmedium 1856 so data related to the forces on the second body piece 18134can be sent to the processor 18144. At least one pole 1848 is associatedwith each chim 18136.

As shown, the first body piece 18132 includes two poles 1848 for eachchim 18136. The poles 1848 are positioned below the chims 18136 suchthat the strain gauges 1852 will measure forces exerted on the chim18136 positioned above. Referring to FIG. 18A, the strain gauges 1852are positioned at different orientations to allow the strain gauges 1852to gather force information along different directions. More straingauges 1852 will allow more data to be gathered regarding the forces onthe chims 18136. Ultimately, the number of poles 1848 and strain gauges1852 used depends on the particular application.

A transmitter (not shown in FIGS. 17-19) may be mounted within theprocessor 1744, 18144. The transmitter may be adapted to take the datacollected from the sensors 1746, 1852 by the processor 1744, 18144 andsend the data to a remote receiver. Preferably, the receiver willanalyze the data and provide feedback to help determine the propersizing of a trial insert or force monitoring system enclosure, as morefully discussed below. Processor 1744, 18144 may be powered by battery1841. The transmitter used in conjunction with the spacer block may besimilar to the transmitter or transceiver described above in FIGS. 2-3.

In FIG. 20, a spacer block 2060 is shown having a handle 2062. Thehandle 2062 allows for easier manipulation and handling of the spacerblock 2060. The handle 2062 of the spacer block 2060 shown in FIG. 20 isintegrally formed with the spacer block 2060. The handle 2062 includes atransmitter 2064 operatively connected to the processor. The transmitter2064 is adapted to transmit data from the processor to a remotereceiver. Alternatively, the handle 2062 may include a hard wiredconnection 2066 to a receiver 2068 such that data from the processor canbe sent to the receiver 2068, through the handle 2062, as shown inphantom in FIG. 20.

Referring to FIG. 21, a spacer block 2170 is shown having a detachablymounted handle 2172. The handle 2172 and the spacer block 2170 includefeatures to allow an electrical connection therebetween when the handle2172 is connected to the spacer block 2170. Any known electricalconnector that is suitable for this particular application may be used.One such electrical connection is shown in FIG. 21 wherein the handle2172 includes an insert portion 2176, and the spacer block 2170 includesa slot 2178. The insert portion 2176 and the slot 2178 have electricalconnectors that are brought into contact with one another when theinsert portion 2176 is inserted within the slot 2178. This type ofconnection is well known, and is similar to the connection of a powercable to a cell phone or the like. This type of connection could alsoinclude threaded fasteners (not shown) to allow the handle 2172 to besecured to the spacer block 2170 after the insert portion 2176 has beeninserted within the slot 2178.

An alternate type of electrical connection is shown in FIG. 22, whereinthe handle 2272 includes projecting conductors 2280 and the spacer block2232 includes openings 2282 to receive the conductors 2280. Theconductors 2280 may be asymmetrical and rotatable, such that afterinsertion into corresponding shaped openings 2282, the conductors 2280may be rotated by actuating a lever 2284, thereby locking the handle2272 to the spacer block 2270.

As described above, the detachable handle 2272 may also include atransmitter 2174 that is operatively connected to the processor throughthe electrical connection between the handle 2172 and the spacer block2170. The transmitter 2174 is adapted to transmit data from theprocessor to a remote receiver, when the handle 2172 is connected to thespacer block 2170. Alternatively, the handle 2272 may include a hardwired connection 2186 to a receiver 2188 such that data from theprocessor can be sent to the receiver 2188, through the handle 2172,when the handle 2172 is connected to the spacer block 2170, as shown inphantom in FIG. 21.

In FIG. 23, the spacer block 1730, 2060, 2170 is fully shown assembledand disposed in a joint. In FIG. 23, the exemplary joint is a kneejoint. As shown in FIG. 23, the spacer block 1730, 2060, 2170 ispositioned between the femur 2312 and the tibia 2316, the sensors(strain gauges 1852, or load cells 1746) are responsive to the forcesimposed by the femur 2312 upon the chims 1736, 18136. Furthermore, thesensors may provide data in a real-time, or near real-time fashion,allowing for intraoperative analysis of the data. Specifically, theprocessor 1844, 18144 contains a memory for storing the data. Inoperation, the processor 1844, 18144 is adapted to receive, as an input,multiple sensor outputs created by each of the strain gages 1852 or loadcells 1746 in response to forces exerted on the chims 1736, 18136. Theprocessor 1844, 18144 may be coupled to a transmitter 2064, 2174 that isadapted to convert the multiple sensor inputs to a data stream, such asa serial data stream, and transmit the data stream, via wired orwireless connection, to a receiver 2068, 2188 as described above.

As shown in FIG. 24, a computer 24170 having processor 24172 and amemory coupled thereto is in communication with at least one sensor24136, which is embedded within the spacer block 1730. If desired, thecomputer 24170 may communicate with ancillary components 24178, 24180,and 24182, as described in greater detail in applicant's related U.S.Patent Application Pub. No. 2004/0019382 A1. For example, in oneembodiment the output device 24180 may display output plots, images, orother data in terms of a force and position of the force imposed upon ajoint. Further, if desired, optional joint angle sensor 24174 andoptional ligament tension sensor 24176 may be used during the jointreplacement procedure to acquire additional data, as generally describedin applicant's above-referenced application.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

I claim:
 1. A system that gathers information to select a trial insertbetween a femur and tibia of a knee of a patient, comprising: a firstbody piece and a second body piece of a spacer, the first body piecebeing separate from the second body piece, where the first body pieceand the second body piece can be assembled together to form the spacer,the first body piece comprising a plurality of poles integrally formedwith the first body piece and extending relative to an inside surface ofthe first body piece; a plurality of sensors disposed on each respectivepole; a processor positioned between the first body piece and the secondbody piece when the first and the second body pieces are assembled, theprocessor coupled with the plurality of sensors; and a chim removablymountable to an outside surface of the second body piece, the pluralityof poles disposed about and aligned with the chim, and where the chim isconfigured to be mountable to the outside surface of the second bodypiece through projections on an underside of the chim and correspondingrecesses formed in the top surface of the second body piece that alignwith the plurality of poles.
 2. The system of claim 1, where the secondbody piece comprises a plurality of apertures configured to receive theplurality of poles.
 3. The system of claim 1, where the plurality ofsensors are configured to detect a mechanical motion of a respectivepole based on a force exerted on the chim, and where the processor isconfigured to receive a signal from the sensors and provide ameasurement of a magnitude of compression experienced by the pluralityof poles.
 4. The system of claim 3, further comprising a transmitterdisposed between the between the first body piece and the second bodypiece when they are assembled together, and where the processor isconfigured to transmit the measurement of the magnitude of compressionto a remote computer through the transmitter.
 5. The system of claim 1,where the plurality of sensors are configured to detect a mechanicalmotion of a respective pole based on a force exerted on the chim, andwhere the processor is configured to receive a signal from the sensorsand provide a measurement of a magnitude of tension experienced by theplurality of poles.
 6. The system of claim 5, further comprising atransmitter disposed between the between the first body piece and thesecond body piece when they are assembled together, and where theprocessor is configured to transmit the measurement of the magnitude oftension to a remote computer through the transmitter.
 7. The system ofclaim 1, where the plurality of sensors are configured to detect amechanical motion of a respective pole based on a force exerted on thechim, and where the processor is configured to receive a signal from thesensors and provide a measurement of a magnitude of bending forceexperienced by the plurality of poles.
 8. The system of claim 7, furthercomprising a transmitter disposed between the between the first bodypiece and the second body piece when they are assembled together, andwhere the processor is configured to transmit the measurement of themagnitude of bending force to a remote computer through the transmitter.9. The system of claim 1, further comprising a handle mounted to thespacer.
 10. The system of claim 9, where the handle is detachablyconnected to the spacer.
 11. The system of claim 9, where the pluralityof sensors are configured to detect a mechanical motion of a respectivepole based on a force exerted on the chim, and where the processor isconfigured to receive a signal from the sensors and provide ameasurement of the mechanical motion.
 12. The system of claim 9, furthercomprising a transmitter disposed in the handle and coupled to theprocessor, and where the processor is configured to transmit themeasurement of the mechanical motion to a remote computer through thetransmitter.
 13. The system of claim 1, where each of the plurality ofpoles comprise a recessed wall.
 14. The system of claim 13, where a topportion of a pole conforms to a shape of a condyle.
 15. The system ofclaim 13, where a sensor is disposed in the recessed wall of one of theplurality of poles.
 16. The system of claim 1, where the plurality ofsensors are configured to detected a mechanical motion of the spacer.17. A system to plurality of poles integrally that gathers informationto select a trial insert between a femur and tibia of a knee of apatient, comprising: a first body piece and a second body piece of aspacer, the first body piece being separate from the second body piece,where the first body piece and the second body piece can be assembledtogether to form the spacer, the first body piece comprising a pluralityof poles integrally formed with the first body piece and extendingrelative to an inside surface of the first body piece, where a topportion of at least one of the plurality of poles conforms to a shape ofa condyle to mate with an underside portion of the second body piece; aplurality of sensors disposed on each respective pole; a processorpositioned between the first body piece and the second body piece whenthe first and the second body pieces are assembled, the processorcoupled with the plurality of sensors; and a chim removably mountable toan outside surface of the second body piece, the plurality of polesdisposed about and aligned with the chim, and where the chim isconfigured to be mountable to the outside surface of the second bodypiece through projections on an underside of the chim and correspondingrecesses formed in the top surface of the second body piece.
 18. Asystem that gathers information to select a trial insert between a femurand tibia of a knee of a patient, comprising: a first body piece and asecond body piece of a spacer, the first body piece being separate fromthe second body piece, where the first body piece and the second bodypiece can be assembled together to form the spacer, the first body piececomprising a plurality of poles integrally formed with the first bodypiece and extending relative to an inside surface of the first bodypiece; a plurality of sensors disposed on each respective pole; aprocessor positioned between the first body piece and the second bodypiece when the first and second body pieces are assembled, the processorcoupled with the plurality of sensors; and at least two separate chimsremovably mountable to an outside surface of the second body piece,where each of the separate chims is associated with a pole from theplurality of poles, and wherein each separate chim is configured to bemountable to an outside surface of the second body piece throughprojections on an underside of the chim and associated recesses formedin the top surface of the second body piece corresponding with theassociated pole.